The IEEE 802.11 and IEEE 802.15.4 standards, which are recently prevailing wireless communication standards, employ an access method called Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). In CSMA/CA, each wireless terminal performs a carrier sense before transmitting a signal. The wireless terminal, when having detected signal transmission by another wireless terminal, does not transmit its signal. The wireless terminal, when not having detected signal transmission by another wireless terminal, transmits its signal.
Consider a plurality of wireless terminals that are remotely located from each other and are difficult to communicate with each other. Each of the wireless terminals is difficult to detect a signal transmitted by another terminal. As a result, one wireless terminal may transmit a signal at a timing at which another wireless terminal is transmitting its signal. In this case, a plurality of signals can arrive at a single timing at an access point that receives signals from a plurality of wireless terminals, so that a collision occurs between the signals. The portion at which the collision occurs between the signals results in degraded reception quality, so that decoding may become difficult.
Wireless sensor networks (WSNs), in particular, typically involve low antenna gain of wireless terminals. Thus, the carrier sense does not work well at signal transmission and signal collisions tend to occur at the access point.
If the signals that collided with each other can be separated and decoded at the access point, throughput can be improved and retransmission of a signal from a wireless terminal can be prevented to achieve power saving of the wireless terminal.
One known technique to separate the collided signals at the access point uses channel information of each of the collided signals. If the channel information of each of the collided signals can be estimated, the collided signals can be separated from each other.
One channel information estimating technique calculates, as will be described below, a “preamble correlation value” that represents a correlation value between a preamble at the start of a signal and a known preamble sequence.
Specifically, as illustrated in FIG. 1, while a preceding signal arrives at an access point, a subsequent signal arrives at the access point following the preceding signal and a collision between the preceding signal and the subsequent signal occurs at the access point. FIG. 1 is an example diagram of a collision between the preceding signal and the subsequent signal. The preceding signal and the subsequent signal each have a packet format illustrated in FIG. 1, for example, and include a preamble (PA), a header, data, and a cyclic redundancy check (CRC) bit. Assume that the subsequent signal arrives with a delay of a preamble length or more from the start of the preceding signal. Specifically, while the preamble of the preceding signal does not overlap the subsequent signal, the preamble of the subsequent signal overlaps the preceding signal.
Let S1(t) be the preceding signal illustrated in FIG. 1, let h1 be channel information of the preceding signal, and let n(t) be noise. Then, a received signal y(t) at the access point at the preamble portion of the preceding signal is expressed by expression (1), where y(t) is the received signal at a time-of-day t, S1(t) is the preceding signal at the time-of-day t, and n(t) is noise at the time-of-day t.y(t)=h1S1(t)+n(t)  (1)
Let N be a preamble length. Then, a preamble correlation value K1 of the preceding signal is calculated using expression (2) and the preamble correlation value K1 can be regarded as the channel information h1 of the preceding signal. Specifically, when the preamble of the preceding signal does not overlap the subsequent signal, the preamble correlation value of the preceding signal is directly an estimated value of the channel information of the preceding signal.
                                                                        K                1                            =                            ⁢                                                                    1                    N                                    ⁢                                                            ∑                                              t                        =                        0                                                                    N                        -                        1                                                              ⁢                                                                                  ⁢                                                                  h                        1                                            ⁢                                                                        S                          1                                                ⁡                                                  (                          t                          )                                                                    ⁢                                                                                                    S                            1                                                    ⁡                                                      (                            t                            )                                                                          *                                                                                            +                                                      n                    ⁡                                          (                      t                      )                                                        ⁢                                                                                    S                        1                                            ⁡                                              (                        t                        )                                                              *                                                                                                                          ≈                            ⁢                              h                1                                                                        (        2        )            
Let S2(t) be the subsequent signal illustrated in FIG. 1 and let h2 be channel information of the subsequent signal. Then, the received signal y(t) at the access point at the preamble portion of the subsequent signal is expressed by expression (3). Thus, a preamble correlation value K2 of the subsequent signal is calculated using expression (4). S2(t) is the subsequent signal at the time-of-day t.
                              y          ⁡                      (            t            )                          =                                            h              1                        ⁢                                          S                1                            ⁡                              (                t                )                                              +                                    h              2                        ⁢                                          S                2                            ⁡                              (                t                )                                              +                      n            ⁡                          (              t              )                                                          (        3        )                                                                                    K                2                            =                            ⁢                                                                    1                    N                                    ⁢                                                            ∑                                              t                        =                        0                                                                    N                        -                        1                                                              ⁢                                                                                  ⁢                                                                  h                        1                                            ⁢                                                                        S                          1                                                ⁡                                                  (                          t                          )                                                                    ⁢                                                                                                    S                            2                                                    ⁡                                                      (                            t                            )                                                                          *                                                                                            +                                                      h                    2                                    ⁢                                                            S                      2                                        ⁡                                          (                      t                      )                                                        ⁢                                                                                    S                        2                                            ⁡                                              (                        t                        )                                                              *                                                  +                                                      n                    ⁡                                          (                      t                      )                                                        ⁢                                                                                    S                        2                                            ⁡                                              (                        t                        )                                                              *                                    ⁢                                      (                    t                    )                                                                                                                          ≈                            ⁢                                                                    1                    N                                    ⁢                                                            ∑                                              t                        =                        0                                                                    N                        -                        1                                                              ⁢                                                                                  ⁢                                                                  h                        1                                            ⁢                                                                        S                          1                                                ⁡                                                  (                          t                          )                                                                    ⁢                                                                                                    S                            2                                                    ⁡                                                      (                            t                            )                                                                          *                                                                                            +                                  h                  2                                                                                        (        4        )            
Because the preceding signal interferes with the preamble of the subsequent signal, the term of the preceding signal is left in the preamble correlation value K2 of the subsequent signal as depicted in expression (4). Thus, accuracy in estimating the channel information of the subsequent signal is degraded.
To enhance the accuracy in estimating the channel information of the subsequent signal, one possible approach is to remove the preceding signal from the received signal. To achieve this purpose, a directivity pattern that forms a null in a direction of arrival of the preceding signal is formed using null-steering that uses an antenna array including a plurality of antenna elements. The directivity pattern is formed by multiplying the received signal by a weight.
Examples of related-art are described in Japanese Laid-open Patent Publication No. 2007-282120. Further, examples of related-art are described in “Applications of Space Division Multiplexing and Those Performance in a MIMO Channel”, IEICE Trans. Commun., vol. E88-B, No. 5, May 2005, pp. 1843-1851.
The following is a preamble correlation value calculated with respect to the received signal multiplied by the weight (hereinafter may be referred to as a “weight-multiplied signal”). The weight-multiplied signal here is a result of removal of the preceding signal from the received signal by null-steering.
Specifically, let Nr be the number of antenna elements that constitute the antenna array, let h1 (Nr×1 vector, [h11, h12, . . . , h1Nr]T) be channel information of the preceding signal corresponding to each of the antenna elements, let h2 (Nr×1 vector, [h21, h22, . . . , h2Nr]T) be channel information of the subsequent signal corresponding to each of the antenna elements, and let W (1×Nr vector, [W1, . . . , WNr]) be a weight. Then, the weight-multiplied signal is expressed by expression (5). It is noted that the channel information and the weight are expressed by vector, and that the channel information may be called a “channel vector” and the weight may be called a “weight vector”.Wy(t)=(w1h11+ . . . +wNrh1Nr)S1(t)+(w1h21+ . . . +wNrh2Nr)S2(t)+Wn(t)   (5)
Because the multiplication of the received signal y by the weight W forms a null in the direction of arrival of the preceding signal, the term of the preceding signal in expression (5) is substantially 0. Thus, the weight-multiplied signal can be expressed by expression (6).Wy(t)=(w1h11+ . . . wNrh1Nr)S1(t)+(w1h21+ . . . wNrh2Nr)S2(t)+Wn(t)≈(w1h21+ . . . wNrh2Nr)S2(t)+Wn(t)  (6)
A preamble correlation value K of the weight-multiplied signal given in expression (6) is calculated using expression (7). The weight-multiplied signal corresponds to the received signal including the preceding signal and the subsequent signal, from which the preceding signal is removed. Thus, the preamble correlation value K calculated using expression (7) corresponds to the preamble correlation value of the subsequent signal.
                                                        K              =                            ⁢                                                                    1                    N                                    ⁢                                      Σ                    ⁡                                          (                                                                                                    w                            1                                                    ⁢                                                      h                            21                                                                          +                                                  …                          ⁢                                                                                                          ⁢                                                      w                            Nr                                                    ⁢                                                      h                                                          2                              ⁢                                                                                                                          ⁢                              Nr                                                                                                                          )                                                        ⁢                                                            S                      2                                        ⁡                                          (                      t                      )                                                        ⁢                                                                                    S                        2                                            ⁡                                              (                        t                        )                                                              *                                                  +                                                      Wn                    ⁡                                          (                      t                      )                                                        ⁢                                                                                    S                        2                                            ⁡                                              (                        t                        )                                                              *                                                                                                                          =                            ⁢                                                                    w                    1                                    ⁢                                      h                    21                                                  +                                                      w                    2                                    ⁢                                      h                    22                                                  +                …                +                                                      w                    Nr                                    ⁢                                      h                                          2                      ⁢                                                                                          ⁢                      Nr                                                                                                                              (        7        )            
In expression (7), while the weight W and the preamble correlation value K are known values, pieces of channel information h21, h22, . . . , h2Nr are unknown values. Given the pieces of channel information h21, h22, . . . , h2Nr that exist in number corresponding to the number of the antenna elements Nr, the preceding signal and the subsequent signal that collide with each other can be separated from each other.
However, the calculation of the preamble correlation value of the subsequent signal using the weight-multiplied signal as depicted in expression (7) makes it difficult to uniquely determine Nr pieces of channel information h21, h22, . . . , h2Nr of the subsequent signal due to the only one equation available for Nr unknown values. As described above, despite the multiplication of the received signal by the weight performed for enhancing the accuracy in estimating the channel information of the subsequent signal, the very multiplication by the weight makes it difficult to uniquely determine the channel information of the subsequent signal. The difficulty in uniquely determining the channel information of the subsequent signal makes it difficult to separate the preceding signal and the subsequent signal that collide with each other from each other.